It may seem to you that Microsoft is 10 steps behind and that they suddenly came from nowhere to become a "leader", but the fact is that their antivirus software has been around and working well for years.
Security Essentials is the combination of two efforts within the company that came together: OneCare (consumer protection) and ForeFront (enterprise/corporate protection).
OneCare was ridiculed by many people, but it turned out to be among the elite in terms of accuracy of its scanning engine. By elite, I mean that under testing of the industry standard virus-testing organization (can't remember the name) it was in the top-5 worldwide.
Turns out that much of the ridicule was ginned up by the big-name "security software" companies who did not like that Microsoft was putting out a cheap product that was better than their scanners, and did not have the effect of slowing down peoples' PCs the way their overly-bulky software did.
Most people haven't heard of ForeFront because it is aimed at companies to protect their various servers from threats. (I have used it on my servers for years.)
The end result of the combination of products was Security Essentials for consumers and ForeFront for companies, but they are the same exact scanning engine underneath. In fact, the interfaces are pratically identical on them. So when you install Security Essentials, you are using the same exact software and virus definitions that companies around the world are using to protect their critical servers.
The only real difference between the packages is that ForeFront can be monitored and controlled within a corporate domain centrally, whereas Security Essentials is a stand-alone product.
The fact that Microsoft put it out there as a free product is the right thing to do, but it really is amazing if you think about it.
OF COURSE Microsoft needs anti-virus protection. If Apple ran 90% of the world's computers and Microsoft had a mere 5% share, don't you think the shoe would be on the other foot? Apple would be targeted by everyone and suffering these same issues, and Microsoft would be sitting back not doing much -- like Apple is today. It's hard to be the leader in this case, and it's great to see that Microsoft has stepped up with excellent software.
It may sound like I'm being a salesman for Microsoft, but my passionate descriptions of this software are simply from my own life experience. For several years I went from software package to software package looking for virus protection that would give me good protection, not slow my PC down, not annoy me with constant reminders and "alerts" about ridiculous "evil cookies", and constantly try to scare me into upgrading every year to their next release.
I tried everything out there, and I mean everything. I came close a few times to finding something I liked, but then I found it did not work for one reason or another. It didn't matter if it was free or paid software, they all disappointed me.
Then I tried Microsoft OneCare, and that was the closest thing to my ideal anti-virus that I came across. It was not free software at the time -- I think it cost around $50 after a free trial. I did the free trial and was so happy with it that I purchased it, and then installed on my various PCs. It made me a little nervous because of the snarky comments from people on the Internet about it, but then I started seeing the test results coming out showing that it was among the best in the world at finding viruses.
When OneCare eventually become Security Essentials -- and became free -- it made a good thing even better and simpler.
If you're using Trend Micro and it works for you, then great! Everyone has their own list of ideal qualities for virus protection, so use whatever works for you.
I recommend Security Essentials because I know that so many people struggle with protecting their PCs from online threats, and the security software business is vulture-like in its drive to scare people and extract their money for bloated junk that makes them wrongly feel that cookies are as bad as viruses and they have to keep paying every year for basic protection. And many times those software packages have terrible detection rates!
As a web site owner, every day I have to cope with people asking me "why can't I log in", and it turns out that their "security" software is blocking their browser's ability to store the login cookie. Why does it block the cookie? Because paid "security" software has to act like it is constantly doing something critically important , or else why would you bother paying for it every year?
That is angering to me, and it is a sign of very bad software practices. Security software should stay in the background at all times, silently doing its thing, and only make its presence known when something bad happens. It should not interfere with browsing the web, aside from protecting against true threats. It should be a small, efficient software package that does not consume more than a minimum amount of memory and processing power.
So that's why I recommend Security Essentials.